This invention relates to composite blades for use in fluid flow machines and, more particularly, to improving the shear strength characteristics thereof.
In recent years, significant advances have been made in developing composite blades for fluid flow machines, such as gas turbine compressors and fans, by making use of structural composite reinforcements having high strength characteristics. Generally, the major portion, or primary structure, of the blade comprises substantially parallel laminates of small diameter reinforcing filaments, having high strength and high modulus of elasticity, embedded in a lightweight matrix which is generally extremely weak compared to the longitudinal strength of the filaments (typically only one to five percent as strong). These laminates, possessing essentially unidirectional strength characteristics, are laid up at specified predetermined angles to each other, and to the blade longitudinal axis, and the matrix cured to create a rigid structure. For example, the blade can be made strong in tension longitudinally and chord-wise by suitably orienting the fibers in each laminate. In embodiments involving predominantly nonmetallic materials, the blades comprise graphite filament laminates in an epoxy resin, though any fiber embedded in any binder, such as an organic resin, may be employed. Further, the structures may also comprise any metallic system including boron filaments in an aluminum matrix.
One factor which has discouraged the introduction of composite blades into operational service in aircraft gas turbine engines is their vulnerability to what is commonly referred to as foreign object damage. Many types of foreign objects may be entrained in the inlet of a gas turbine engine, ranging from large birds such as eagles, to hailstones, sand and rain. While the smaller objects can erode the blade materials and degrade the performance of the fan or compressor, impact by the larger objects may cause more severe damage. Under large impact loads, composite blades severely distort, twist and bend developing high localized multidirectional stresses. These may result in portions of the blade being torn loose or in extensive delamination of the filament laminates. A contributing factor is that the laminated composite blade is very weak in tension perpendicular to the plane of the blade (i.e., across the airfoil portion from pressure to suction surface), and weak in resisting shear loads between the laminates. In these types of loadings, the loads are carried entirely by the matrix which, as noted above, is extremely weak compared to the filaments.
Several approaches have been considered in an effort to improve the transverse and interlaminar shear strength of composite blade airfoils and, thus, improve their impact tolerance. These approaches have primarily involved selecting the proper filament/matrix system and processing the material in a manner so as to optimize their load-carrying potential. While moderate progress has been made, it is apparent that the foreseeable structural materials may not afford adequate transverse shear capability without a change in the structural configuration. Thus, it becomes desirable to develop a composite blade for turbomachinery application which does not rely entirely on the matrix properties for resisting transverse shear loads.